SESSION:26

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1)       Explain the significance of symmetrical components in real time power system analysis, apart from which they are said to be designed for analyzing unbalanced system?

Without symmetrical components, it would be very difficult to analyze unbalanced loading, the effects of harmonics, and fault studies.

Also the impedances of loads are dependent on the sequence networks.

For example, the positive sequence a, negative sequence and zero sequence impedance networks of a synchronous machine are all different from each other.

In a real power system, there is very rarely a situation where a perfectly symmetrical and balanced system exists.

2)       Do the sequence networks of synchronous machines vary with the fault?

The sequence impedances of a generator are different during fault conditions; there is the sub-transient impedance (stage 1), transient impedance (stage 2) and synchronous impedance (stage 3).

3)       Is there any relation between the fault and the occurrence of several sequence networks? 

Any power system during a fault can be analyzed with 3 equivalent sequence networks: positive, negative and zero sequences. And there are 3 different equivalent circuits for each sequence.

4)       And how can the protective equipment be able to differentiate between the different sequences currents?

Modern numerical protection relays calculate the sequence currents and voltages from the CT and VT inputs just like someone would calculate it by hand.

5)       What is the use of VFD and tell about its working.

We could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since we can operate the fan at any speed below its maximum, we can vary airflow by controlling the motor speed instead of the air outlet damper.

6)       What is Skin Effect?

Property of a conductor by virtue of which a line offers different resistances at various frequencies of AC. Thus , a dc resistance of any conductor is not constant always , rather it is quite different from its high frequency resistance. This occurs due to difference in flux linkage with the outer surface and deep inside the conductor.

7)       How will you find the transmission line voltage by observation?

We can tell the Transmission line voltage by observing

·         Level of insulation used

·         Thickness of conductors

·         No. of conductors

·         No. of parallel conductors of one phase (for 400 kV it’s 2 and for 765 kV it’s 4 conductor for one phase)

All the above factors can give us rough idea of voltage for which the line is designed. Further, there are very few national standards which will help us reaching close to the accurate answer.

8)       Why do we not use frequency below 50 Hz and what will happen?

Going much below 50Hz would cause incandescent lights to flicker. The threshold for humans to perceive such flicker is typically about 16Hz, though it can be detected at higher frequencies by some people.

If the frequency is increased substantially, inductive loads become very high impedance, requiring higher voltages to drive the same amount of current. Considering motors (which are inductive loads) use something like half the world's electricity supply, this would be a bad thing. Also, transmitting them across large distances becomes a problem due to the effects of inductance of long lines. Further, higher frequencies require generators (with a given number of poles) to rotate faster. Going too high increases wear and tear, in addition to adding mechanical instability. The 50 to 60Hz region provides a good compromise. Why 60Hz is used in some parts of the world (primarily USA) while 50Hz is used in others is because of historical artefacts that are not really cost effective to remedy.

That said higher frequencies allow the use of lighter transformers and smaller motors and lower frequencies reduce inductive effects over long lines. Higher frequencies (such as 400Hz) are used for specialized purposes (such as in aircraft, where weight is important and you don't have to worry about long lines) and lower frequencies are used in some railway traction systems.

9)       Does the reverse saturation current flow in the reverse direction of the conventional current in a diode under reverse biased condition?

If we keep the normal convention of voltage v_D and current i_D directions in a diode, the reverse current is negative. The usual electrical equation of the diode is

i_D=I_S[exp(v_D/(ηV_T))−1]i_D=IS[exp⁡(v_D/(ηV_T))−1]

Where i_S, η and V_T are diode parameters.

If v_D is negative (diode with reverse voltage) then the exponential is approximately zero and we can write

i_D≈−I_S

This is the negative value of current seen in the example picture below, marked as "Reverse Current". If the reverse voltage gets too large, then the diode enters in breakdown and usually blows out (unless it is prepared to do it in a controlled manner, such as it happens with Zener diodes.)

10)    Why surge impedance loading does not change with compensation?

The surge impedance loading (SIL) of a line is the power load at which the net reactive power is zero. So, if your transmission line wants to "absorb" reactive power, the SIL is the amount of reactive power you would have to produce to balance it out to zero. You can calculate it by dividing the square of the line-to-line voltage by the line's characteristic impedance.

So compensation is designed while taking into consideration the SIL.

Hence, with change in compensation, the SIL will change (& vice-versa).

11)    What is Surge Impedance Loading? Derive it.

The surge impedance loading or SIL of a transmission line is the MW loading of a transmission line at which a natural reactive power balance occurs.  

Transmission lines produce reactive power (Mvar) due to their natural capacitance. The amount of Mvar produced is dependent on the transmission line's capacitive reactance (XC) and the voltage (kV) at which the line is energized.  In equation form the Mvar produced is:

  

Transmission lines also utilize reactive power to support their magnetic fields.  The magnetic field strength is dependent on the magnitude of the current flow in the line and the line's natural inductive reactance (XL).  It follows then that the amount of Mvar used by a transmission line is a function of the current flow and inductive reactance.  In equation form the Mvar used by a transmission line is:

 

A transmission line's surge impedance loading or SIL is simply the MW loading (at a unity power factor) at which the line's Mvar usage is equal to the line's Mvar production.  In equation form we can state that the SIL occurs when:  

If we take the square root of both sides of the above equation and then substitute in the formulas for XL (=2pfL) and XC (=1/2pfC) we arrive at:  

The term  in the above equation is by definition the "surge impedance”. 

12)    Is a transformer neutral to be grounded? When can earthed cables be used? When can the un-earthed cables be used?

Neutral grounding of most generators and transformers are so designed to identify and control fault currents involving ground (i.e., L-G and L-L-G faults) as it affects the zero-sequence current. As the question is about transformers it is the practice nowadays to have all the EHV and UHV transformers effectively grounded so that the line voltages of healthy phases during a L-G fault does not exceed 80% of L-L voltage.

To assist in the process of insulation coordination, Basic Insulation Levels (BIL) has been recommended. Insulation of Lightning Arrestors (LAs), transformers, switches, CBs, bus bars etc must be able to withstand a surge of this value with a factor of safety. Thus BIL value decides the required rating of LAs, and insulation of transformers, switches, CBs, bus bars etc, both against surge and power frequency voltages. This BIL value depends on the status of grounding.

For example, a 220 kV system will have a BIL of 900 kV if system is effectively grounded and 1050 kV if non-effectively grounded and consequently the required insulation against surge voltages for transformers may be 1050 kV or 1300 kV (next standard BIL values) respectively. The insulation of switches and CBs, bus-bars are also affected in a similar manner.

13)    What are the factors affecting Corona? What is the advantage and disadvantage of Corona?

Factors affecting Corona:

·         Atmosphere- Corona depends on the physical condition of atmosphere. In a rainy or stormy weather corona will be more at less voltage.

·         Space between the conductor- Spacing between the conductor is also depend on the corona more space in between the conductor less electrostatic  effect and less corona.

·         Line voltage- Line voltage is also affect on corona, greater the line voltage more corona will be formed.

·         Surface of the conductor- Irregular and rough surface have more corona than the smooth surface.

Advantages of Corona:

·         Due to formation of corona surrounding conductors starts conducting; hence the diameter of the conductor is increased.

·         The effect of transient produced by surges is also reduced.

Disadvantages of corona:

·         It increase line loss and decrease transmission efficiency.

·         Non sinusoidal voltage drop across the line as the current drawn by the corona is non sinusoidal.

·         Corrosion of conductor may cause due to production of ozone gas.

Methods of reducing corona effect:

·         By increasing conductor spacing

·         By increasing conductor size.

14)    What are V and inverted V curves?

V Curve is Excitation versus Armature current curve. V curve is the graph showing the relation of armature current as a function of field current in synchronous machines. The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied.

Inverted V Curve is Excitation versus Power Factor curve.

The synchronous motor “V Curves” shown below illustrate the effect of excitation (field amps) on the armature (stator) amps and on system power factor. There are separate “V” Curves for No-Load and Full-Load and sometimes the motor manufacturer publishes curves for 25%, 50%, and 75% load. Note that the Armature Amperage and Power Factor “V” Curves are actually inverted “V’s”.

Assume it is desired to determine the field excitation which will produce unity power factor operation at full motor load. Draw a line from the unity power factor (100%) operating point on the Y-axis to the peak of the inverted Power Factor “V” Curve (blue line). From this intersection, project down (red line) from the full-load unity power factor (100%) operating point to determine the required field current on the X-axis.

In this example the required DC field current is shown to be just over 10 amps. Note at unity power factor operation the armature (stator) full-load amps are at the minimum value.

Increasing the field amps above the value required for unity power factor operation will cause the machine to run with a leading power factor, while field weakening caused the motor power factor to become lagging. When the motor runs at either leading or lagging power factor, the armature (stator) amps increases above the unity power factor value.

 

15)    What is meaning of 5P20 in Current Transformer (CT)?

CT's are categorized as Protection CT, Special Protection
CT and Measuring CT. Based on this, the CT's are classified. Here is the meaning of the CT classes:

Class 5P20:

The letter 'P' indicates it is a protection CT.

The number 5 indicates the accuracy of the CT. Most common  accuracy numbers are 5 and 10.

The number 20 (called accuracy limit factor) indicates that  the CT will sense the current with the specified accuracy  even with 20 times of its secondary current flows in the  secondary. This is required for protection CT, because the fault current is high and the CT should be able to sense  the high fault current accurately to protect the system.  The common numbers are 10, 15, 20 and 30.

Class PS:

PS is for 'Protection Special'. This class of CT's are used  for special protection such as differential protection,  distance protection etc.

Class 1M:

The letter 'M' indicates it is a measuring CT.

The number 1 indicated the accuracy of the CT. The measuring CT's should be more accurate than the protection CT. The most common accuracy numbers are 0.5 and 1.